1. Field of the Invention
The present invention relates to an image processing unit, an image processing method, and an imaging device incorporating the image processing unit.
2. Description of the Related Art
Shot noises or dark current noises occur in an image sensor of a digital camera or an imaging circuit of an imaging device such as a scanner due to its property, and deteriorate image quality. To improve the quality of captured images, image processing is needed to remove or reduce noise components from the images.
An image processing unit performing image processing is comprised of, for example, a circuit to execute filtering with use of a digital filter. Certain frequency components can be reduced by such filtering to remove noise components. However, by simple filtering an important portion of an image such as edges needed for a user to perceive an image may be lost. That is, simple filtering may cause deterioration in image quality.
There is a known filtering with a ε-filter which does not deteriorate image quality unlike simple filtering. The ε-filter is a filter to determine, as a similar pixel, a pixel whose divergence is equal to or lower than a threshold in an arbitrary filter size. The ε-filter can be repeatedly used since a window size can be reduced. Thus, the entire image or an arbitrarily divided image block can be repeatedly subjected to filtering while changing the number of filterings depending on a certain condition such as brightness of a subject, as disclosed in Japanese Patent Application Publication No. 2008-205737, for example.
However, through repeated filterings with the ε-filter, the pixels of the edge portion of an image and neighboring pixels of the edge portion become similar. This results in generation of an image with an expanded and blurred edge portion.